Communication satellite



Feb. 11, 1989 J. c. YATER COMMUNICATION SATELLITE Filed April 22, 1965 Ra om mm WC (K A .V 5 z ATTORNEYS United States Patent 3,427,623COMMUNICATION SATELLITE Joseph C. Yater, 1706 St. Marks Place, Fairfax,Va. 22030 Filed Apr. 22, 1965, Ser. No. 449,934 U.S. Cl. 343-705 Int.Cl. H01q [/28, 15/20 7 Claims ABSTRACT OF THE DISCLOSURE the gravitygradient.

This invention relates to communication satellites and more particularlyto a passive communication satellite. According to the invention, ahighly directional antenna array is placed in orbit around the earth andis employed as a passive communication satellite, and functions as aneflicient reflector of transmitted electromagnetic signals such ascommunication signals.

The antenna array consists of spaced antenna elements, each of which isextended and stabilized in space, utilzing the gradient of the earthsgravitational force field. This gradient or spatial change in directionand strength of the gravitational force provides forces for stabilizingan axis of the satellite with respect to the plane of the orbit of thesatellite, and this gradient further provides a force used, inaccordance with the discovery of this invention, in extending flexiblefilaments in space. The gravity gradient force extends and maintains inextension each antenna array in a direction normal to the surface of theearth, i.e., towards the center of the earth. This extension providesthe major gain of the antenna array so that the antenna pattern of thecomplete antenna array will have its narrowest lobe in planes containingthe major dimension of the antenna and its broadest lobe structure inthe directions defining the surface of an imaginary cone having aconstant angle with respect to the axis of the major dimension of theantenna array.

The antenna array of this invention exhibits the ability to steer or,synonimously, to direct, the reflected electromagnetic signals by simplyvarying the frequency of transmission. This steering or directing isderived from the regular spacing of reflecting elements positioned alongthe major or earth pointing axis of the satellite. Thus, by using acertain frequency of transmission, the signal can be reflected back,i.e., back towards the earth, at a desired angle with respect to themajor axis of the satellite in order to be received at a desired locusof locations on the earths surface. The supporting structure of theelements of the array consists of a light-weight, flexible structuresuch as a thin filament made of plastic or glass. In a preferredembodiment, the passive satellite of this invention consists of asingle, flexible filament of nonconducting material such as plastic orglass. Spaced regularly thereon, and functioning as reflectors of thetransmitted electromagnetic energy, i.e., the signal, the surface iscoated with a conducting material to thereby define a series of bands.The bands are spaced a distance apart coresponding to approximatelyseven-tenths of the wave length of the transmitting frequency, ormultiples thereof. The antenna is made of very small diameter filamentso that a 1000 foot long antenna weighs only a very small fraction of apound.

In order to discuss the advantages available by using this inventionover current approaches, it is helpful to discuss the present state ofthe art of communication satellites. Such satellites are under extensivestudy and experimentation, and large programs are under way usingdifferent approaches. Both passive and active approaches are beinginvestigated and used. Passive satellites do not require power andmerely reflect the transmitted electromagnetic energy back to earth.Active satellites, which require power, amplify the transmitted signalbefore relaying the communication signal back to earth.

An example of a passive satellite system that has shown goodcapabilities is called Project West Ford. In this case the individualsatellites, which are merely dipoles weighing a few micrograms each, areplaced in an orbit at an altitude of about 2,000 miles above the surfaceof the earth. Abuot 480 million of the dipoles, weighing altogetherapproximately 44 pounds, were actually placed into orbit using only onelaunching vehicle to dispense them into orbit. This orbiting dipoletechnique can provide continuous worldwide coverage with highreliability and high traffic handling ability, but the transmission lossof the system is large enough to thereby give rise to a low informationrate, unless large ground equipment is used. In addition, specialmodulation-detection techniques are required to reduce the short termeffects of the random motion of the dipoles in orbit. Also, such anorbiting belt of dipoles must be carefully observed and studied toinsure that other radio services, scientific studies, and spaceexploration are not subjected to interference.

The active communication satellite system approach is also underintensive investigation and already hundreds of millions of dollars havebeen spent by governmental agencies alone on efforts to develop a systemof this type. This approach has the advantage that the amplification ofthe signal in the satellite reduces the transmission loss over that ofthe passive approach, as in Project West Ford. But on the other hand therequirement for amplification reduces the capacity of the system beyondthat of the Project West Ford approach, since the amount ofcommunication signals that can be relayed through the satellite islimited by the bandwidth of the amplifier and also by the poweravailable from the satellite. This makes the problem of sharing thesatellite system among many users quite complex. The Comsat Corporationis considering the feasibility of either or both of a medium altitude(5,000 miles) or a synchronous altitude system (18,000 miles). NASA hasalready helped launch several medium altitude satellites (Telstars,Relays) and three synchronous altitude satellites (Syncoms). But allapproaches so far considered require satellite systems costing many tensof millions of dollars with the potential cost to each user necessarilyvery high.

By the practice of the present invention the high capacity andreliability of the P roject West Ford is realized, but without the largetransmission losses, special modulation-detection techniques, ordiflicult noise problems. In addition, this is done at an extremely lowcost, compared to the active satellite systems. These and otheradvantages, such as providing much higher jamming immunity of providingnavigation signals to a wide class of users, result from the practice ofthis invention, making it feasible to construct high gain, highlydirectional antenna arrays in space at low cost and of low weight.

In the high gain, directive antenna of this invention, the signalreflected from each reflector along the axis of the antenna is in phase,at the receiver, with the transmitted signal. This increases thereflected power in the desired direction proportional to the square ofthe numbers of reflectors, i.e., their second power. For

randomly spaced reflectors, such as those employed in Project West Ford,the power reflected in the desired direction is only proportional to thenumber of reflectors, i.e., their first power. So, for a given number ofdipoles in a stabilized directive array as compared to the same numberof randomly oriented and randomly spaced dipoles as in Project WestFord, the reflected power in the desired direction according to thepresent invention is increased by a factor that is larger than thenumber of dipoles used. As an example, one filament of this inventioncontaining 10 bands, i.e., dipoles, has the ability to return a signalof strength as could be returned from all 10 dipoles of the Project WestFord, if the latter dipoles were all concentrated in one region andcould be illuminated in one antenna beam width. Or, to state thecomparison in other terms, the 10 dipoles of the antenna element of thisinvention can return a signal approximately 10 times stronger than thereflected signal from the Project West Ford dipoles if the 60 foottransmitting antennas of Project West FOPd are used in both cases atranges to the dipoles of 3,000 miles.

As far as the active satellites so far launched are concerned, all havevery little antenna gain, so that the signal that can be returned froman .antenna array of this invention is much larger for the aboveexample, where the 20 kw. 8 kmc., Project West Ford transmitter and the60 foot dish antenna are used. This signal strength is 100 or more timeslarger than the current and programmed active satellite systems canprovide.

In addition, this passive directional antenna array can handle largenumbers of users, as the number of users that can employ the system on anon-interfernig basis is a function of the gain of the antenna array,and if the users are appropriately spaced, hundreds of thousands ofusers can transmit and receive via the same array. This high capacity ofthe system is further enhanced by the lightweight and inexpensive natureof the antenna arrays. Separate groups of the filaments may be made tohandle separate frequency bands in order that a wide class of users canbe handled.

In the drawings:

FIGURE '1 is a schematic representation of a single antenna array ofthis invention in orbit around the earth.

FIGURE 2 is a view of a single antenna array.

FIGURE 3 is a schematic showing of the relationship between an imaginarycone of transmission and its corresponding imaginary cone of receptionfor the antenna array shown in FIGURE 1.

FIGURE 4 is a view, similar to FIGURE 2, of a modification.

FIGURE 5 is a view of a further modification.

FIGURE 6 is a view of a further modification.

Referring now to FIGURE 1 of the drawings, the numeral 10 denotes asingle filament of glass or plastic forming a directional antenna arrayby coating the surface with a conducting material 1 2 at periodicintervals denoted by D. This periodic requirement is imposed in order toenable the reflected signals trom each reflector 12 to be in phase atthe receiver. The required spacing D of the reflectors 12, which may beconsidered dipoles at the signal frequency, and the relative directionsfrom the reflectors 12 to the transmitter and receiver, is given by theequation cos a+cos 8 wherein D is the spacing along the filament betweenthe center of the reflectors 12 as shown in FIGURE 2, and

n is an integer A is the signal wavelength at, B are the angles betweenthe straight orbiting filament 10 and the directions to the transmitterand the receiver,

respectively, as shown at FIGURES 1 and 3.

Referring now to FIGURE 3 of the drawings, the zone Z represents thelocus of points on the surface of the earth (sh-own hat for ease inrepresentation) for which a transmitter defines the angle at of FIGURE 1with respect to the filament (antenna array) 10. The zone Z thenrepresents the locus of points on the surface of the earth of allreceivers which define the angle ,6 of FIGURE 1. The zones areilllustrated as having appreciable width for purposes of illustration.

From a consideration of the above explanations, it will be seen that atransmitter placed anywhere within zone Z will obey the relation n)\=lD(c0s al+cos ,9) with respect to a receiver located anywhere within zoneZ The ability to steer or direct the transmitted signals is derived fromthe regular spacing of the reflecting bands or dipoles of the antennaarray. From the expression n)\=D(c0s a-j-cos 8), it is seen that for afixed angle a (a fixed transmitter), a change in wavelength of thetransmitted signal will cause a change in 8, and hence a change alongthe earths surface for those points Z on which constructivereinforcement of the signal will occur. Assuming a case for the firstorder, i.e., 11: 1, for an orbital altitude of 3,600 miles for theantenna array and an initial angle 3 of 30 degrees, a change of 50megacycles from an 8 ki-lo-megacycle transmission frequency will cause achange of radius on the zone Z of the order of magnitude of miles.

In practice a ring of filaments is placed in orbit or a Syncom typeorbit may be employed. In the former case, when one filament passes outof the position shown in FIGURE l with respect to the transmitter andreceiver, another will move into its place. In the latter case, thefilament remains fixed, as shown in FIGURE 1.

The length of each coated band and the spacing therebetween is made tocorrespond with the desired altitude and operating frequency.

This relationship holds for microwave antenna lengths of less thanseveral thousand feet in low altitude orbits, and for longer arrays athigh altitudes or lower frequencies. For longer arrays than theselimits, the curvature of the phase front of the transmittedelectromagnetic wave at the larger values of 0; causes deterioration inthe theoretical gain of a microwave array.

This limit on the antenna length for any given a and p can be removed bychanging the spacing of the reflecting elements 12 so that the array isfocused to receive or transmit using the given pair of angles, a and 5.A several fold increase in allowable length can be obtained for a usefulrange of values of a and B by adjusting the spacing for a midpoint valueof a and p3 of the given range of values of a and 3. But this adjustingof the spacing of the reflectors will be necessary only for extremelyspecialized applications, since the gain of a 1,000 foot microwaveantenna is already large enough to meet nearly all of the usualcommunication or signal relaying requirements.

This limitation on length only applies with respect to maintaining an inphase relation in a given direction for signals of a given frequency.For many applications, it will be feasible to lengthen the filament toallow the same filament to be the supporting structure to the requirednumber of arrays with each array having a different spacing D, Thiswould enable the arrays on the filament to transmit between atransmitter and a receiver as many channels of information at as manycarrier frequencies as there were separate arrays on the one filament,i.e., arrays each having a ditferent D. By this means, one filamentcould meet any bandwidth requirement.

An important exception to this limitation of length of an array occurswhen the requirements of the communication system are that a single wideband is more useful than the equivalent total bandwidth made up ofseveral smaller bandwidth arrays. For this case at the cost of a loss ingain of the array of 25% from that of the limited length maximum gainarray, the array can be made as toward the center of the earth by virtueof the gradient 1 of the earths gravitational field. The maximum tensionin the antenna filament 10, due to the gravity gradient, is reduced fromthe magnitude of the force of gravity on the filament at the orbitaltitude by a factor proportional to the ratio of the length of thefilament to the distance from the center of the filament to the centerof the earth.

The deployment of the filaments is accomplished by dispensing thefilament coil into space :with the filament being held in place on alightweight spool during this launching phase by being covered with asubliming material such as napthalene. After the coil is dispensed fromthe launch vehicle the napthalene sublirnes, allowing the filament tounwind from the spool, one turn at a time.

The thickness of the filament may be reduced to make the payload weighta negligible factor, as a glass fiber .002 cm. in diameter will yield anadequate bandwidth capability to the array. A 1,000 foot filament ofthis thickness Weighs less than M of a pound. On the other hand, addedthickness to the filament may be added if a longer life expectancy tothe filament is required, But, at a less weight penalty, the lifeexpectancy can also be increased using redundant spaced filamentsconnected together at convenient intervals along the filament.

An embodiment of the invention is illustrated at FIG- UR-E 4, showing aflatted filament or ribbon 20 of insulating material, providingadditional protection against micro-meteorite damage when exceptionallylong life is required. This ribbon is also provided .with conductingelements 12 regularly spaced therealong at a distance D apart. Thereflecting bands 12 of FIG. 4 form horizontal dipoles at right angles tothe axis of the flattened filament. These horizontal dipoles may be usedto give a greater reflected signal level for small angles for a or B.

A technique to reduce any bending resulting from solar radiation is tomaintain a constant rotation of the antenna array or 20 about its axis.A constant rotation may be maintained using solar radiation pressure onsolar paddles. These paddles, denoted by the numeral 30 of FIGURE 5, areattached to the antenna array and cause rotation by using the familiarprinciple of coating opposite surfaces thereof with reflecting andabsorbing materials, respectively. The rotation rate may be governed byemploying paddles having coiled, flexible ends 34, as shown at FIGURE 6.The unrolling of the coils due to the centrifugal force as the rotationing the ends 34 to unroll. As the coils unroll, surfaces coated similarto the surfaces on the opposite side are exposed so as to slow down therotation rate.

The relative size of the paddles, as shown, will ordinarily be muchsmaller. This is because spinning would take place if only a very smallturning torque existed. To get an estimate of the magnitude of torquerequired, a continuous torque of 3X 10* dyne centimeters would result inan increase of 1 radian per day in the rotation rate of a 1 mil diameterfilament 3000 feet long. This torque would result from an uncompensatedsolar pressure acted on a reflecting surface A that had a movement arm Lwith the following value for the product of L and A:

This value of the product would exist from a 10 sq. cm. area with amoment arm equal to the diameter of the 1 mil filament. Then, therequired unbalanced solar presrate increases caus- 5 one side andblackened on the other side and the mirror image of this area was placedon the other side of the filament with the surfaces of all areas beingat temperatures that maintained thermal equilibrium for the environmentof the solar rflux on a spinning filament.

In addition to mechanical schemes used to provide damping to thespinning torque, of which the technique illustrated in FIGURE 6 is anexample, electromagnetic damping torques may also be used. A simpletechnique is to make electric generators out of a very small fraction ofthe dipoles 12 by removing the coating in small vertical strips from themiddle of both sides of a dipole to thereby define a conducting loop,which generates an electromotive force in the magnetic field of theearth proportional to the spin rate. To limit the spin rate to onehundred revolutions per second with an applied solar torque of 3 10-dyne centimeters, approximately one hundred dipole conducting loopswould be adequate. This amount of spin would be much too small to haveany measurable elfect in disturbing the equilibrium of the filament. Theextremely small eifect of this spin upon the filament motion resultsfrom the extremely small moment of inertia about the spin axis.

This spinning technique is a preferred technique to minimize thermalbending, first since the spinnnig will be difficult to prevent, secondbecause spinning about the vertical axis in a vertical field of forcewould tend to straighten out any residual bending forces, as these wouldbe minimized by the averaging effect of the spinning motion, third aninitial spin could exist for many cases. The many cases for which aninitial spin would exist would occur when the filament had beenpreviously twisted prior to being placed on the spool before thedispensing process. This twisting could be done to insure that any setor creep that occurred while the filament was in place on the spool wasreduced to a minimum.

Another technique that may be used to reduce thermal bending of theantenna array when heavier filaments or ribbons are used, is to haveflexible coupling at regular spacings along the vertical axis of thearray or much thinner filaments connecting segments of the antennaarray.

For the user with both high signal level and'wide information bandwidthrequirements, these requirements can be met through using additionalfilaments with each filament having a difierent spacing betweenreflectors. These additional arrays are placed in similar orbits or canbe placed on the same filament so that the transmitter beam illuminatesthe required number of filaments so that the required signal level andbandwidth is obtained.

For extremely high signal level requirements, but with lower bandwidthrequirements, a satellite system is designed so that many antennas withthe same reflector spacing D may be in the beam width at the same time.In this case, the signals may be the total of the randomly phasedreflected signals of all the illuminated antenna arrays when appropriatesignals of a and t! are used. This increases still further the increasein the signal level than is available from the Project West Fordtechnique.

In order to show some of the advantages and capabilities of the arraysof this invention, it is necessary to compute the signal power andbandwidth obtainable from these arrays as well as the improvement in thesignal power obtainable from these arrays compared to the signal powerfrom active satellites. First, then the power from a passive satelliteinto a receiver is given by:

P PtGQGfAf4L (41rR (41R?) where P is the transmitted power G, is thetransmitter antenna gain G is the gain of the filament array A is theeflective area of the antenna for the receiver sure would result if this10- cm. area was silvered on A is the effective area of the filamentarray 7 R is the distance to the filament from the transmitter R is thedistance to the receiver from the filament.

For the case where the passive satellite is the filament consisting ofthe array of vertical dipoles, we have 2L sin {3 where B, as in FIGURE2, is the angle between the filament and receiver and L is the length ofthe filament.

Also

where D and D are the diameters of the parabolic antennas:

For this case then:

.056P.D D L sin a sin 8 h R Rg Since the curvature of the wavefront cancause an unwanted difference in phase of regularly spaced scatterers,then this unwanted difference in phase can result in a limit to thelength of the array. Using as a limit that for the worst geometry caseat a given altitude, the ends of the array can be excited at no morethan 90 difference in phase from the center of the array, the followinglimit in length as a function of altitude is imposed:

where h is the altitude of the orbit, R is the radius of the earth.

The worst case for which this maximum length is computed is for ahorizon transmission or reception from the array. Then using this lengthand considering a horizon to horizon transmission, the followingequation results for the power at the receiver:

Then computing the following ratio of P to KT, where KT=.4 10 when T is290 K., for the following assumed values:

D =D :30 feet Pt=10 kw. 7\=3.75 cm.

Gives and For example, for an orbit with an altitude of 2,000 miles itcan be seen that 1.4 megacycles of information can be sent over theassumed link with a 20 db power ratio between the signal and the noisepower from a 290 K. source over the same bandwith. The curve can bescaled to give the capabilities of other communication links.

For transmissions between 6 foot antennas, the above ratios would bedecreased by 625 so that for this link a two-way voice channel could bemaintained with the above 20 db ratio between signal and noise andbetween locations 6,700 miles apart. If the power of the transmitter wasreduced to 1 kW., then the ratio between the signal power and the noisepower at 290 would be reduced to 10 db. For 100 watt transmitter asignal of 240 cycles per second would give the same signal to noisepower ratio over the 6 foot antennas.

Using the West Ford 20 kw. transmitters and 60 foot antennas wouldprovide a 35 db signal to noise ratio for the 1.4 mc. bandwidth ofsignal and an even higher signal to noise ratio if the West Ford lownoise receivers were used.

Although the signal to noise power in the above case is sufficient, thebandwidth of information that a single antenna array can transmit mustalso be considered. This bandwidth of information is a function of alength of the array and for the case where the maximum length is usedthe following relation is obtained:

The maximum bandwidth for the maximum length array for a 2,000 milealtitude orbit is 360 kc. The 1.4 m. bandwidth of information couldstill be transmitted over one filament array using the assumed link withthe 30 foot antennas or using a West Ford link if proper coding wasused.

However, without using this coding, it would be necessary to utilizemore than one filament of this array length to transmit this signalbandwidth. In this case the required number of four filaments for thetransmission of the 1.4 mc. signal bandwidth could be within thebeamwidth of the transmitter for the link with the 30 or 60 foottransmitting antenna,'if sufficient number of arrays had been placedinto one ring with random spacings or if the required number of fourarrays had been placed on one filament. By placing several thousand ofthese filaments into a ring, then with a very high probability, thetransmitter beamwidth from a 30 or 60 foot antenna can be used to locatealong the satellite ring a region where 4 or more arrays can be used totransmit the required information.

After the glass or plastic filaments are drawn, the conductivereflective bands may be placed thereon following conventional chemical,electrical and vapor plating procedures.

If chemical plating is employed, satisfactory results have been obtainedby using a solution of palladium chloride and/or palladium chloride andtin chloride containing about 3 to 4 grams of reagent per gallon. ThePdCl or the mixture of PdCl and SnCl provides a uniform coating of thereducing agent on desired portions of the fiber. Following the coatingof the elements with the solution of the reducing agent an aqueoussolution of the salt of the metal to be plated onto the coated surfacesis applied to the fibers. A very satisfactory metallic coating has beenprovided by using a plating solution of grams of CuSo -5H O per liter ofsolution. Such a solution will have a pH of from about 5.7 to 6.3 with aspecific gravity of 1.10 at 70 F. It has also been found that uniformityof the coating is enhanced by adding to the solution a 1% solution ofsodium hydroxide. In general, 5 to 15 minutes in such a plating bathwill provide a uniform metallized coating on the glass surfaces.

It has been found that coatings of this nature generally have a veryuniform thickness and that a metal layer is generally deposited to athickness of about /3 micron and further immersion of the element in theplating solution will generally not increase the thickness of the metallayer.

From the above, it is seen that the ability to steer or to direct thetransmitted signals stems from the regular spacing of the reflectingbands or dipole elements, and expressed by the relation n)\=D (cosa-i-cos 19). It is to further noted that the capture and continuingorientation of a filament by the gravity gradient so that it is alwayspointing towards the center of the earth, thus utilizing this relation,requires a minimal length of approximately ten feet.

What is claimed is:

1. A passive communication satellite system comprising, in orbit aroundthe earth, an elongated filament, said elongated filament being of aninsulating material and provided with a plurality of spaced reflectingelements along the longitudinal axis thereof at regular intervals, saidfilament being oriented so that its longitudinal axis is normal to thesurface of the earth.

2. The system of claim 1 wherein said filament is in the form of a flatribbon.

3. The system of claim 1 wherein said reflecting elements are in theform of bands of a conductive material.

4. The system of claim 1 wherein said filament carries means to rotateit about its axis in response to solar radiation forces.

5. The system of claim 1 wherein the filament is at least ten feet inlength.

6. The passive communication system of claim 1, wherein said elongatedfilament is flexible.

7. The method of establishing a passive communication system comprisingthe step of placing in orbit around the earth a coiled and flexiblefilament of insulating material having a plurality of regularly spacedreflecting elements along the length thereof and allowing thegravitational gradient of the earth to uncoil and thereby elongate thesaid filament to thereby place it in orbit around the earth in anextended configuration with its longitudinal axis directed towards thecenter of the earth.

References Cited UNITED STATES PATENTS 2,624,003 12/ 1952 Iams 343-7853,057,579 10/ 1962 Cutler et a1 343705 3,128,467 4/1964 Lanctot 3437853,165,751 l/ 1965 Clark 343705 3,168,263 2/1965 Kamm 343-705 3,202,9988/1965 Hoffman 343785 3,277,479 10/ 1966 Struble 244-455 2,936,453 5/1960 Coleman.

3,144,606 8/1964 Adams et al 343-909 20 ELI DIEBBRMAN, Primary Examiner.

US. Cl. X.R.

